Process of producing alkali metal amalgams in mobile mercury cathode cells



April 1, 1958 J. CLEMENT 2,829,096

PROCESS OF PRODUCING ALKALI METAL AMALGAMS IN MOBILE MERCURY CATHODE CELL-S Filed July 31, 1956 2 SheetsSheet 1 April 1, 1958 J. CLEMENT PROCESS OF PRODUCING ALKALI METAL AMALGAMS IN MOBILE MERCURY CATHODE CELLS 2 Sheets-Sheet 2 Filed July 31, 1956 United States Patent 9 M PROCESS OF PRODUCING ALKALI METAL AMAL- GAMS IN MQBILE MERCURY CATHODE CELLS Jean Clment, Ixelles-Brussels, Belgium, assignor to Solvay & Cie., Brussels, Belgium, a Belgian company Application July 31, 1956, Serial No. 601,305

Claims priority, application Belgium August 27, 1955 3 Claims. (Cl. 204-124) It is known that in mobile mercury cathode cells for the production of chlorine and alkali metal amalgams, the chlorine produced is accompanied by small quantities of hydrogen which can in some cases be sufficient to form an explosive mixture with the chlorine. The presence of this hydrogen arises from local decomposition of the alkali metal amalgam in the cells.

It is known that these decompositions can be caused by the presence on the amalgam, of graphite particles which come from the anodes, but they may equally be due to the presence of amalgams of other metals in the form of insoluble lumps in the mercury and which are known, on account of their appearance, as mercury lumps.

The latter tend to rise to the surface of the liquid cathode and to collect there in floating blocks, interfering with the electrolysis itself.

In horizontal cells in which the mercury circulates in a relatively thick sheet and the movement is sufliciently turbulent, the bad eifects of mercury lumps are not apparent at first sight and one may assume that hydrogen is evolved, as a result of the mercury lumps, only at the end of the cell, beyond the weir at the end of the cathode sheet proper and where the decrease in circulation speed of the mercury favours the rising and collection of the lumps.

Whenever a systematic analysis is made of the gases along the cell, one concludes that at the extreme outlet end for the amalgam, the gas may be considerably enriched in hydrogen, the presence of which is due not only to mercury immiscible impurities such as graphite debris, but also to mercury lumps which appear at the surface of the cathode before reaching the weir arranged at the end of the cell.

In cells of the vertical type where the cathode streams in a very thin sheet over a fixed support, ending at the bottom in a drain a relatively large evolution of hydrogen can be detected above the drain. This evolution may be explained by the fact that, by reason of the low circulation rate of the amalgam out of the drain, the mercury lumps inevitably present in the amalgam have time to rise to the surface of contact between the electrolyte of the cell and the amalgam, and to cause the decomposition of the latter.

It is the object of the invention to eliminate this unwanted and dangerous evolution. To this end a suitable stirring or agitation of the liquid cathode is'ensured at the places where circulation of that liquid becomes too slow: in this way the mercury lumps are kept in dispersion throughout the depth of the liquid cathode and their bad efiect is thus neutralised. The applicants have observed in a vertical laboratory cell, possessing a mercurycollecting vessel at the bottom of the vertical streaming surface of the cathode, that without agitation of the mercury in the vessel the content of hydrogen of the gas in the immediate proximity of the vessel is of the order of by volume; whereas when this mercury is submitted 2,829,096 Patented Apr. 1, 1958 to suitable agitation, the hydrogen content is lowered to 0.5 to 0.6%.

In the application of the process according to the in vention one'may use any known method of agitation. It is not necessary to employ complicated devices whose introduction into the cell could be a nuisance in practice and could introduce serious difficulties. The most simple apparatus to which preference is given by virtue of its suflicient effectiveness, consists for example of a gear pump of which the intake branches on the outlet channel of the amalgam and of which the output opens some millimetres below the surface of the amalgam in the collecting vessel of the cell. The agitation circuit is superimposed on the normal mercury circulation circuit.

The accompanying drawings illustrate schematically two embodiments of the process forming the object of the invention.

Figure 1 shows a perspective view of the arrangement applied to a vertical cell; Figure 2 shows an arrangement correspondingly applied to a horizontal cell.

According to Figure 1, the mobile mercury cathode of a vertical cell flows out of a weir device 1 and streams down the length of the conducting vertical support 2 disposed between two anode plates 3 and 4; at the bottom of the support 2, the mercury containing in solution the amalgam formed in the course of the electrolysis is collected in a trough 5 from where it runs out through a tube 6 to an apparatus for decomposing the amalgam. The required agitation is obtained by a secondary circuit comprising the branches 7 through which part of the liquid cathode is drained by the pumps 8 with'motors 9 and returned by pipes 10 into the collecting trough 5 at a level slightly below the mercury surface. For the sake of clarity in the drawing, the electric current connections to the cathode and anode, the electrolytic liquid and the outer casing of the cell, and the anodic gas exhausts have not been represented.

The apparatus for carrying out the process according to the invention is not limited to the particular representation of the drawing. Thus for example the agitation may be obtained by a single pump with an inlet branched from pipe 6 and an outlet feeding a collector with a number of branches leading to the collecting trough 5.

Figure 2 shows an example of the application of the principal of the invention to a horizontal cell. Only the elements necessary for the easy explanation and understanding of the invention are given, the cell itself being represented only by the conducting plate 12 and lateral walls 11 interrupted for clarity. The mobile cathode constituted by mercury containing alkali metal amalgam in solution and unwanted lumps of amalgam in suspension, circulates from the base plate 12 towards the endpiece provided with a drain 14 into which the mercury spills over and from which it leaves by tube 15. By

reason of the decrease in circulation speed, the lumps tend to rise towards the surface and collect there. This inconvenience is avoided by the agitation circuit comprising a tube 16 disposed between the bottom of the drain 14 and the pump 17 driven by motor 18. The agitation circuit is completed by two return pipes 20 from the pump 17 into the drain 14 by way of openings 19 arranged below the level of the mercury in the drain.

The agitation apparatus does not have the object of separating the mercury lumps, since one seeks on the contrary to diffuse them in the mercury present in the cell. Separation is then carried out in a separate part of the apparatus away from the electrolytic cell. This separate zone which can preferably form one body with the cell works as a decanting vessel, the lumps rising to the surface, collecting there and being then capable of removal by any suitable known means.

Care must be taken to avoid any introduction of aqueous solutions into the decanting vessel; the mercury lumps are in the presence of amalgam rich in alkali metal and the simultaneous presence of water would lead to premature decomposition of the amalgam. The process of the present invention therefore offers the advantage of avoiding decomposition of the amalgam in the electrolytic cell by mercury lumps and in consequence the production of hydrogen and corresponding loss of alkali metal.

I claim:

1. In the electrolysis of alkali metal chlorides in an electrolysis zone having a moving mercury cathode to form alkali metal amalgam and chlorine, the steps which comprise accumulating the mercury and associated amalgam as a body at the amalgam discharge end of the electrolysis zone in an accumulation zone, continuously withdrawing a portion of said body of mercury from said accumulation zone, and continuously returning at least some of said portion as a continuously flowing stream directly into said body of mercury contained in said zone, at a speed sufiicient to maintain continuous agitation in said body of mercury effective to maintain any lumps of amalgam formed in the course of electrolysis in a dispersion in said body of mercury and to suppress separation of said lumps from said body of mercury, whereby the generation of hydrogen is suppressed.

2. In the electrolysis of alkali metal chlorides in an electrolysis zone having a moving mercury cathode to form alkali metal amalgam and chlorine, the steps which comprise accumulating the mercury and associated amalgam as a body at the amalgam discharge end of the electrolysis zone in an accumulation zone, continuously withdrawing a portion of i said body of mercury from said accumulation zone, and continuously pumping at least some of said portion as a continuously flowing stream directly into said body of mercury contained in said zone below the surface thereof at a speed sutlicient to maintain continuous agitation in said body of mercury effective to maintain any lumps of amalgam formed in the course of electrolysis in a dispersion in said body of mercury and to suppress separation of said lumps from said body of mercury, whereby the generation of hydrogen is suppressed.

3. In the electrolysis of alkali metal chlorides in an electrolysis zone having a moving mercury cathode to form alkali metal amalgam and chlorine, the steps which comprise accumulating the mercury and associated amalgam as a body at the amalgam discharge end of the electrolysis zone in an accumulation zone separate from said electrolysis zone, continuously withdrawing a porticn of said body of mercury from the bottom of said accumulation zone, and continuously returning at least some of said portion as a continuously flowing stream directly into said body of mercury contained in said zone below the surface thereof at a speed sufiicient to maintain continuous agitation in said body of mercury effective to maintain any lumps of amalgam formed in the course of electrolysis in a dispersion in said body of mercury and to suppress separation of said lumps from said body of mercury, whereby the generation of hydrogen is suppressed.

References Cited in the file of this patent UNITED STATES PATENTS 2,323,042 Honsberg June 29, 1943 2,351,383 Wolf June 13, 1944 2,733,202 Boyer Ian. 31, 1956 2,744,864 Muller May 8, 1956 

1. IN THE ELECTROLYSIS OF ALKALI METAL CHLORIDES IN AN ELECTROLYSIS ZONE HAVING A MOVING MERCURY CATHODE TO FORM ALKALI METAL AMALGAM AND CHLORINE, THE STEPS WHICH COMPRISE ACCUMULATING THE MERCURY AND ASSOCIATED AMALGAM AS A BODY AT THE AMALGAM DISCHARGE END OF THE ELECTROLYSIS ZONE IN AN ACCUMULATION ZONE, CONTINUOUSLY WITHDRAWING A PORTION OF SAID BODY OF MERCURY FROM SAID ACCUMULATION ZONE, AND CONTINUOUSLY RETURNING AT LEAST SOME OF SAID PORTION AS A CONTINUOUSLY FLOWING STREAM DIRECTLY INTO SAID BODY OF MERCURY CONTAINED IN SAID ZONE, AT A SPEED SUFFICIENT TO MAINTAIN CONTINUOUS AGITATION IN SAID BODY OF MERCURY EFFECTIVE TO MAINTAIN ANY LUMPS OF AMALGAM FORMED IN THE COURSE OF ELECTROLYSIS IS A DISPERSION IN SAID BODY OF MERCURY AND TO SUPPRESS SEPARATION OF SAID LUMPS FROM SAID BODY OF MERCURY, WHEREBY THE GENERATION OF HYDROGEN IS SUPPRESSED. 